1. Field of the Invention
The present invention generally relates to trays, a tray construction, and methods of using trays. More particularly, the present invention relates to a drying or dehydration trays and their methods of use, for example, in drying produce such as fruits or vegetables.
2. Description of Related Art
Between 1910 and 1920, L. N. Miller invented a box-like dryer for drying produce. The dryer was heated artificially by oil and included a large fan capable of high air velocity, humidity shutters, and bleeder vents. This was the predominant design for dryers through the 1940s and spawned many variations.
In the 1960s, scientists at the University of California at Davis developed the now common overhead return “Tunnel Dryer.” Variations of this design are now in use throughout the U.S. and overseas. When using the Tunnel Dryer, products to be dried are placed on wooden trays measuring 3 feet×6 feet that are stacked 40 trays high onto rail carts. The carts are wheeled into the tunnels for processing.
The Four Phases of Hot Air Dehydration
There are typically four phases in the hot air dehydration of produce. The first phase is known as “raising the core temperature.” In the first phase of raising the core temperature, the product is warmed as fast as possible, without case hardening the product to within 10 to 20 degrees of the process air temperature. In the field of the invention, “case hardening” is an undesirable thermal treatment or over heating of the product whereby the permeability of the surface or skin of the product, for example, fruit, is decreased or eliminated. For example, case hardening can undesirably seal the outer surface of a product and prevent moisture from escaping from the product during drying. In the counter flow configuration, the wet fruit is placed in the cool end and is subjected to very wet air that has lost 20 degrees or more by passing through the length of the Tunnel. This wet air transfers heat very fast and as the cart moves forward in the dryer, the process air temperature rises and humidity drops. This accelerates the transition to the second phase.
In the Parallel flow configuration, the wet cart is placed in the hot end and the product is immediately subjected to the high temperatures and low humidity of the high-pressure end of the tunnel dryer. Rather than pulling the cart with the product when it is dry (counter flow), parallel flow requires that in less than two hours another cart must be placed in the hot end of the tunnel to prevent the produce on the previous cart from case hardening. Thus, the wet product drives the dehydration process rather than the dry product. As each cart is placed in the high-pressure end, a charge of wet cool air bathes all of the cars behind it for a few minutes. This dehydration and re-hydration cycle continues throughout the process.
In the second phase of hot air dehydration, that is, “Rapid Dehydration,” the moisture content of the product is in near free fall. To maximize production, moisture inside the dryer needs to be controlled. As a rule, the moisture content of the process air when drying most products, measured at the high-pressure end, should be 17% to 19%. After the air passes through the dryer, the relative humidity at the cool end should be between about 35% and 50%. Since each product is different, the processing conditions for each product may vary.
In the third phase of hot air dehydration, that is, the “Transition,” is the most critical phase in dehydration, in regards to possible damage to the product. The high rate of moisture release experienced in the second phase slows down to a crawl. Most of the water in the product is substantially gone. Capillary action at the cellular level now provides the majority of the free water being driven off. The evaporative cooling that has kept the core temperature of the product well below the process air temperature slows as well. Case hardening, cooking, and carmelization are all very possible as the product passes through the transition phase.
In the fourth and final phase of hot air dehydration, that is, the “Bake Out,” is characterized by a slow reduction in the product moisture content. This phase is normally the longest and, depending upon the target moisture content, may include over half the dwell time. Carmelization is still a threat in the last phase as well.
Batch Drying
Of the three ways to use a Tray Dryer, “Batch Drying” is the simplest and most commonly used. Batch drying refers to the loading of the tray dryer with all of the product-laden wooden or stainless steel trays and carts at one time and drying the lot, without moving the carts within the dryer. While some products react well to this procedure, most do not. The loss of the even and consistent dehydration quality motivates most operators to investigate other drying protocols. The problem with batch drying is in the lack of uniformity of the environment the product is exposed to. Since the leading edge of the leading car “sees” (or is exposed to) a much different environment than that of the trailing cart, significant differences in moisture content can occur within the product. It is like drying the same product in two different dryers, each set at a different temperature.
Existing drying trays have been traditionally made from laminated, composite or hardwood with the following problems: wooden trays can absorb water, juice, flavors, and odors; wooden trays can harbor bacteria, microbes, nematodes, black algae, fungus, and other potential contamination; wooden tray's splinters and chips can contaminate the food product; wooden trays are hard to clean and products can be damaged and deformed when being scraped from the tray surface; wooden trays need to be screwed or nailed together, such fixtures can easily work free and contaminate products or jam handling systems; wooden trays can absorb heat quickly and burn or discolor the surface of the fruit that lays against the wood.
One alternative to wooden trays are stainless steel trays. Stainless steel trays are somewhat uncommon due to high cost, hard to release surface, high heat transfer burning fruits, and less than optimal footprint (smaller sized due to handling weight).
Due to the disadvantages of existing wood and metal tray technology, a need exists for providing a produce drying tray with improved performance and reliability than existing trays.